As we learned on yesterday’s daf the Sages reject Rabbi Yehuda’s suggestion that we should be able to conclude what was sold by looking at the agreed upon sale price. The Gemara on today’s daf asks why this would not negate the sale because of the laws of ona’a.
Ona’a is a biblical prohibition (see Vayikra 25:14), which forbids someone to take unfair advantage of another by overcharging or undercharging when negotiating a business deal. There are three different levels of ona’a established by the Sages:
1. When the ona’a – the amount that is overcharged – must be returned
2. When the ona’a is so small that we assume the parties do not care about it, and it does not have to be returned
3. When the ona’a is so large that the entire transaction is nullified.
The amount that is set by the Mishna in Massekhet Bava Metzia (4:3-4) as ona’a is one-sixth of the value of the transaction. Therefore, if the overcharge is exactly one-sixth, that money is returned; if it is more than one-sixth the transaction is nullified; if it is less than one-sixth the transaction stands and the money need not be returned. The Mishna teaches that there is a time limit on demanding a return of the ona’a – the amount of time that the purchaser needs to show the object to a knowledgeable friend or businessman. If he holds it for longer than that period, we assume that he has accepted the price, and that there is meḥila – that he “forgives” any money that is owed to him.
Our Gemara concludes that the rule of ona’a does not apply in this case because the difference in price is so great that it cannot be assumed to be a mistake. The Sages’ argument is that the purchaser may have agreed to pay more than the value of the object simply because he wanted to give a present to the seller.